With rapid advance of manufacturing technologies in thin film transistor LCD (TFT-LCD), and the advantages of slim and light, power saving and low radiation, LCDs have been widely used in various types of electronic products such as personal digital assistant (PDA), notebook computers, digital cameras, video cameras, mobile phones, and the like. In addition, the industry has invested heavily in research and development and uses mass production facilities. As a result, quality of LCD continuously improves and the price drops relentlessly, application scope of LCD is expanded drastically. However LCD cannot generate light by itself. It requires a backlight module to accomplish display function.
Refer to FIG. 1 for the cross-sectional view of a conventional backlight module. The backlight module 10 is generally designed for a large size LCD. It includes a diffusing plate 11, a plurality of lamp tubes 12, a reflecting plate 13, a frame 14 and a plurality of films 15. The diffusing plate 11 is a semi-transparent acrylic plate which has a light incident surface (lower surface), a light exit surface (upper surface), and a plurality of diffusion granules located therein (not shown in the drawing) for diffusing lights. The lamp tubes 12 are evenly located below the light incident surface of the diffusing film 11. The reflecting plate 13 is located under the lamp tubes 12. A major portion of lights emitted from the lamp tubes 12 directly enters the diffusing plate 11, and the rest portion of the lights are reflected by the reflecting plate 13 and then enter the diffusing plate 11. The frame 14 encases the diffusing plate 11, the lamp tubes 12, and the reflecting plate 13 so as to protect the elements located in the backlight module 10. Several optical films 15 are further located above the diffusing plate 11 to output lights in a uniform fashion.
In the conventional backlight module, all lamp tubes are positioned under the diffusing plate. The diffusing plate usually has a thickness between 2 and 4 mm, the light path from an incident surface to an exit surface is too short. In addition, the scatting times of light scattering by the diffusion granules in the diffusing plate is limited. As a result, the diffusing plate cannot contribute uniform lights. Moreover, the lamp tubes are packed inside the backlight module in the conventional display so the heat dissipation of the backlight module becomes poor. Heat generated by the lamp tubes tends to accumulate inside the backlight module and transmits through the diffusing plate to the LCD located thereabove. Liquid crystals in the LCD are then affected. Furthermore, the diffusing plate has a higher shielding ratio which reduces luminous flux. This also affects the brightness of the backlight module and the quality of the LCD.
Therefore to overcome the disadvantages occurred to the conventional backlight modules and to improve the backlight module to achieve more uniform lighting, greater luminous flux and improved heat dissipation so that yields and quality of the LCD can be improved is a goal hotly pursued in the industry.